isolating vehicle batteries

[Cjhstang]

I'm looking at running a three battery setup. I want to isolate my 2 starting batteries from the rest of the truck.
the running/glow plug battery (and electric preheaters for future wvo & winter biodiesel) will be run from a deep cell battery (maybe two).
my question is how can i isolate the starting batteries so that i can attach the alternator to them and charge them when the alternator is already connected to the deep cell battery and rest of truck electric system?

[Believer45]

Do a google search with

battery +isolator

and you will come up with many links to electronic isolators and suppliers.

Hellroaring Technologies

Power Stream

ASE Supply

It looks like you will need to decide how many batteries and amperage before you buy the isolator.

Dave / Believer45

[Cjhstang]

Thanks!
i was looking at isolators. i can't figure out what amperage rating i would need on an isolator. would i need to get an isolator with the same rating as my alternator OR would i need an isolator relevant to the expected amperage draw from the two starting batteries while charging?
and of course my next question following is, what is the maximum amperage draw that a starting battery can peak when charging?

[Larry S.]

You only need to isolate the third battery. I'd use a relay isolator rather than an electronic as the electronic isolators all have a voltage drop that limits charge rate. The stock trailer towing package isolator should do your job well.

[Albin]

Originally Posted by Larry S.

You only need to isolate the third battery. I'd use a relay isolator rather than an electronic as the electronic isolators all have a voltage drop that limits charge rate. The stock trailer towing package isolator should do your job well.

Agree. I'd use the STancor 586-903 15 vdc relay, perfect for this application.

Good luck.

Al

[drcampbell]

Originally Posted by Cjhstang

Thanks!
... i can't figure out what amperage rating i would need on an isolator. would i need to get an isolator with the same rating as my alternator OR would i need an isolator relevant to the expected amperage draw from the two starting batteries while charging?
... what is the maximum amperage draw that a starting battery can peak when charging?

The maximum charging amperage a battery can draw is about the same as the maximum cranking amperage it can deliver. (not exactly, but close enough for this discussion) In practical terms, that means a fully-discharged battery can absorb everything the alternator can deliver.

But the starting-lighting-ignition battery in a vehicle is almost never fully discharged and it's not a steady-state condition when it is. It might accept 100 charging Amps the moment you start the the engine, but the charge current starts going down as its state-of-charge goes up.

-

Installing a battery isolator with a capacity equal to the alternator capacity will always be adequate. But as you've already surmised, a smaller isolator might also be adequate if the alternator's seldom delivering its maximum rated output.

But if you weren't considering adding electrical demand, we wouldn't be having this discussion, and the price difference between an isolator rated for 100% of the alternator's rating and a smaller one isn't very much, especially after considering the installation cost.

-

If you plan frequent deep discharges of the deep-cycle battery, either a relay isolator or a simple two-diode isolator may prevent the SLI batteries from being fully charged. When the deep-cycle battery is switched in, the alternator voltage will drop as it delivers its full output current.

[THEBUNDO]

Originally Posted by drcampbell

The maximum charging amperage a battery can draw is about the same as the maximum cranking amperage it can deliver. (not exactly, but close enough for this discussion) In practical terms, that means a fully-discharged battery can absorb everything the alternator can deliver.

Doug, by "Draw", do you mean "accept as a charge"? If so, I don't see how that is related in the real world (your second comment "that means a fully-discharged battery can absorb everything the alternator can deliver"). What if you have a massive battery (ies) and a small alternator, or vica versa?

[drcampbell]

Yes, that's what I mean.

In a typical vehicle installation, there's an alternator capable of delivering 100-200 Amps and a battery capable of delivering ~500 cold-cranking Amps (at -18 degrees C) or ~1000 Amps at room temperature.

When fully discharged, That same battery can accept a charge current equal to about -1/3 of its rated cranking amps.
(the difference between quiescent voltage and charging voltage is about -1/3 the difference between quiescent voltage and cranking test voltage)

So the battery can accept a charging current equal to everything the alternator can deliver. Sort of like filling a swimming pool with a garden hose - the hose delivers whatever it can deliver, even though the reservoir could accept much more.

If you have massive (or many) batteries and a small alternator, it's the same thing. The alternator delivers whatever current it's capable of, and the batteries fill up with charge at that rate. (after deducting the hotel load)
(note that while adding battery capacity will cause them to reach 100% state-of-charge more slowly, you'll have the same number of Coulombs -- and the same probability of starting an engine -- in the batteries after any given length of charging time)

With a large alternator and small batteries, the voltage regulator would just dial back the alternator's output current to the optimum charging current needed by the battery.
This is the normal everyday operating condition with a fully-charged (or nearly-fully-charged) battery. A fully-charged 100 Amp-hour battery needs only a few Amps to maintain a full charge.

Keep in mind that none of this is steady-state. If you put 100 Amps into a fully-discharged battery, it doesn't stay fully discharged for long. It'll reach nominal terminal voltage pretty soon, at which point the voltage regulator dials back the alternator's output current. As you continue putting Coulombs into the battery, its quiescent voltage and charge impedance rise and it requires less charge current.

[Cjhstang]

Originally Posted by Larry S.

You only need to isolate the third battery. I'd use a relay isolator rather than an electronic as the electronic isolators all have a voltage drop that limits charge rate. The stock trailer towing package isolator should do your job well.

I plan to use the deep cell as the running battery. if i'm using that as the running battery won't it cause problems by isolating it? most of the amp draw will be through this battery.

[Larry S.]

I'm not sure what your reasoning is for doing what you are proposing. I was suggesting a relay that would connect the deep cycle battery in parallel with the starting batteries whenever the engine is running. That's how the trailer towing package is set up on my truck. If for some reason you want to keep your computer and other accessories alive when cranking or other times you may have the starting batteries discharged very low, I could see that condition if you have an electric winch operating from the starting batteries, all that would be needed would be a manual switch to open the relay and disconnect the deep cycle under those conditions. Then when the engine is running and the starting batteries are sufficiently charged energize the relay and your truck will be using all the batteries. If using a diode isolator you may have an issue with maintaining full charge on the deep cycle if all accessories are powered from it and in use, but in most cases there should be no issue.

[Cjhstang]

so i don't have the possibility of the deep cell not charging enough if everything is powered off of it, shouldn't i just put the isolator on the starting batteries instead of the deep cycle?

[Larry S.]

so i don't have the possibility of the deep cell not charging enough if everything is powered off of it, shouldn't i just put the isolator on the starting batteries instead of the deep cycle?

I will back off a bit. Diode isolators isolate both batteries. With an internal regulator on the alternator, like our Fords have, the isolators will drop the charging voltage as much as 0.4 to 0.5 volts on the batteries when high current is being drawn through the isolator. If there is no load on the battery and no load current being drawn from it the voltage drop will go down to somewhere between 0.2 and 0.3 volts. This means the batteries will reach full charge but it will take a longer time. Rvers live with this reduced charging rate all the time but in my opinion it is not the correct way. A relay isolator isolates only the deep cycle/trailer battery when the engine is not running so that the load on the deep cycle does not discharge the starting battery. Give some info on exactly what you are wanting and why. Maybe there is a different and or a better way to achieve what you need. In most cases if the alternator is operating there is no need to isolate batteries. Its only when the alternator is not working/engine not running that isolation is needed.

[drcampbell]

Originally Posted by Larry S.

... With an internal regulator on the alternator, like our Fords have, the isolators will drop the charging voltage as much as 0.4 to 0.5 volts ...

Internal voltage regulators still have external remote voltage sensing. So long as the isolator isn't wired in between the battery and the remote sense line, the regulator will boost the alternator output to compensate for the isolator's voltage drop.

[Larry S.]

This is a good piece of info for me that I was unaware of. Must admit I've not worked much on modern vehicle electrics and a remote sense seems logical. Thats how I would have designed the system. The early internal regulator alternators I worked on did not remote sense.